Jun-Woo Lee

Comparison of liquid crystalline properties of dimeric compounds of different skeletal shapes

Series of dimeric compounds of different skeletal shapes consisting of two triad aromatic ester type mesogenic moieties connected via polymethylene spacers were synthesized and their liquid crystalline properties compared. The two mesogenic units are connected in either Hor T-shape or in linear fashion. In general, it was found that mesophase temperature ranges for the T- and linear-shaped compounds are much wider than for the H-shaped compounds. Moreover, the former are enantiotropic thermotropic materials, whereas the latter tend to be monotropic unless the spacer length is fairly long, i.e. longer than decamethylene. Among the three series, the linearly linked twin compounds had the highest melting and isotropization temperatures. All of the linear and T-shaped dimeric compounds reported in this article form only nematic mesophases.

Comparison of the liquid crystalline properties of dimesogenic compounds bearing alkoxy and perfluoroalkoxy tails

The liquid crystalline properties of two series of nonsymmetric dimesogenic compounds consisting of cholesterol and Schiff base, azobenzene or stilbene mesogens interlinked by a pentamethylene spacer are compared. Compounds in the first series bear the fluorinated OCH2(CF2)6CF3 tail, whereas those in the second series bear the corresponding alkoxy tail. In general, compounds with the fluorinated alkoxy tail exhibited mesophases over a much wider temperature range than those with the alkoxy tail. Moreover, the former favored the formation of smectic phases (either chiral smectic C (Sc*) and SA phases, or SA phase) and suppressed the formation of the cholesteric phase. All of the three compounds of the second series were able to form the cholesteric phase. It was also observed that the layer spacing in compounds with the fluorinated tail was greater by 3 A than those with the alkoxy tail. The compounds that form the Sc* phases exhibited very high spontaneous polarization, 400–500 nC cm−2. The compound with the fluorinated tail has a much shorter response time, 80 µs, than those with the alkoxy tail, 400–500 µs.

Dimeric compounds containing malonic acid as the central linking unit: synthesis and mesomorphic properties

Eight new dimeric compounds containing malonic acid as the central linking unit (series M- n−X) have been synthesized and their mesomorphic properties studied. All these compounds are found to be liquid crystalline, forming mainly nematic and smectic phases. We have carried out microscopic textural observations, X-ray diffractometry as well as solid state NMR studies on these compounds.

Synthesis and properties of combined type liquid crystalline polymers – aromatic polyesters bearing biphenylazophenyl mesogenic pendants connected to the backbone through polymethylene spacers

A series of new combined type liquid crystalline polyesters (LCPs) were synthesized and their LC and other thermal properties were characterized. These polymers consist of poly(oxy-1,4-phenyleneoxyterephthaloyl) backbone onto which the biphenylazophenyl mesogens are attached through the oxyoligomethyleneoxy spacers. The length of the oligomethylene units was varied from the pentamethylene to the decamethylene group. The polymer containing the dodecamethylene spacer was also included in this study. All of the polymers are found to be semicrystalline and they exhibit two thermal transitions before melting. The lower temperature transition occurs between 60°C and 85°C, which decreases with increasing length of the spacers. In contrast, the high temperature transition occurring between 105°C and 117°C depends on the length of spacers to a much less extent. All the polymers form nematic phases in melt. These polymers undergo an initial thermal decomposition around 300°C losing nitrogen gas.

Dimeric liquid crystalline compounds having a central malonic acid moiety: effect of the length of the spacers and terminal chains

We have studied the liquid crystalline properties of two new series of dimeric compounds consisting of a central malonic acid moiety, polymethylene spacers, and Schiffs base mesogens. In the first series we changed the length of the spacers with the terminal chain fixed as the butoxy group; in the second series we changed the length of the terminal alkyl or alkoxy groups while keeping the spacers fixed as decamethylene groups. In the first series, as the length of the spacer increased, the tendency to form smectic phases grew. In the second series, changing the alkyl terminal chain to the alkoxy terminal group raised the melting and isotropization temperatures. Increasing the length of the terminal group diminished the temperature range for the higher temperature smectic phase. The smectic phases are assigned to be smectic A and crystal smectic E phases.

Liquid crystalline compounds having a tribranched structure: substituted malonic esters consisting of two Schiff's base units and a cholesteryloxyalkyl substituent

New tribranched thermotropic liquid crystal compounds were synthesized and their liquid crystalline properties studied by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The compounds are the bis-{10-[4-(4-alkylphenyliminomethynyl)phenoxy]decyl} 2-[6-(cholesteryloxy)hexyl]malonates and the corresponding alkoxy derivatives. These compounds contain three mesogenic units, two identical Schiffs base type mesogens and one cholesteryl either moiety, interconnected in a tribranched structure via spacers. The cholesterly moiety is attached to the malonic acid core through an oxyhexamethylene spacer while the two Schiffs-base moieties are attached through oxydecamethylene spacers. The terminal alkyl group of the Schiffs base unit is either a butyl or decyl group, and the alkoxy terminal group is either a butoxy or decyloxy chain. All the compounds form only an enantiotropic smectic phase, most probably of the smectic C type. The larger spacings determined by small angle X-ray diffraction range from 3.3 to 4.1 nm, which are much shorter than the end-to-end distance (5.9-7.4 nm) of the molecules estimated using molecular models assuming an all trans extended conformation for all the alkyl spacers.

Ferroelectric liquid crystalline polyoxetanes bearing chiral dimesogenic pendants

Two new polyoxetanes bearing dimesogenic pendants were prepared and their liquid crystalline properties were studied by differential scanning calorimetry (DSC), polarizing microscopy, and X-ray diffractometry. The dimesogens consist of cholesterol and biphenyl moieties connected through polymethylene spacers. Their biphenyl ends were attached to the oxetane monomer through a hexamethylene unit and the dimesogenic monomers were subjected to cationic ring-opening polymerization. The molecular weights of the polymers were very low and the degree of polymerization was only about 6. The two polymers were found to be thermotropic and formed only the chiral smectic C (Sc*) phase and, thus, their ferroelectricity was studied. Their spontaneous polarization values at the Sc* phase were about 20 nC cm−2 with responsive times of around 0.3 ms at 7 °C below the clearing temperature.

Cholesterol-based hydrogen-bonded liquid crystals

Hydrogen bonding is a powerful tool for assembling molecules and building new liquid crystalline structures. In this study, non-symmetric dimesogens were prepared by intermolecular hydrogen bonding between rationally designed H-bond donor (3-cholesteryloxycarbonylpentanoic acid) and acceptor (4-(pyridine-4-ylmethyleneimino)phenyl 4-alkoxybenzoate) moieties. Their liquid crystalline properties were investigated by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Cholesteric and smectic phases were observed. As for the covalently linked dimesogens, several types of smectic periodicities occur for these H-bonded cholesteryl compounds depending on the molecular parameters.

SYNTHESIS AND LIQUID CRYSTALLINE PROPERTIES OF T-SHAPED DIMESOGENIC COMPOUNDS

Two series of new dimesogenic liquid crystalline compounds were synthesized and their LC and other thermal properties were characterized. These compounds consist of two mesogenic units connected through polymethylene spacer of varying length in the shape of the letter ‘T’. The difference between the two series is in the structure of tail groups attached on the pendant azobenzene mesogens; one of which is n-butyl and the other is phenyl. The compounds were characterized for their liquid crystallinity by the differential scanning calorimetry (DSC), X-ray diffractometry and visual observation of the melts on a hot-stage attached to a polarizing microscope. All these compounds were found to form only the nematic phase enantiotropically.